Pressure sensitive adhesive (PSA) tape, as well as other adhesive tapes, is often wound in log rolls which are the same length as finished product rolls but have a full web width. These log rolls should be wrinkle and defect free and are lathe slit to the desired product width without rewinding.
Most known log roll winders cut the web while the web is in the air, rather than against a drum. As the knife cuts, it forces the web against a new core. However, as the tail of the web is not supported, the web, particularly thin webs, will not lay down smoothly against the core and wrinkles are produced on the outer wraps. Winders of this type are made by Fuji Tekko.
U.S. Pat. No. 4,775,110 to Welp et al. describes a winding system in which a cutter perforates the web along a line and the web is braked to sever the web. A hot wire can be used as another cutter.
These winders must be stopped while the incoming tape is cut off and started on the core of the next roll. Cutting can not be performed on the fly. In one system attempted by 3M Company, the assignee of this invention, the web is threaded onto a vacuum rotating drum adhesive side out. The web travels with the drum to a wind-up spindle and a cut is made by rotating the knife against the rotating drum while the web is disposed against the surface of the drum. The drum holds the cut ends to prevent wrinkling. This design permits the cut and transfer to be made on the fly without stopping the winding process. This allows the rolls to be wound on line and at machine speeds on a continuous basis.
However, conventional knives having a set range of parameters can not properly cut off adhesive-coated webs against a drum during continuous winding operations without cutting difficulties caused by the adhesive. Three separate and related problems occur. The web and adhesive stick to the knife and wrap on the knife as the knife rotates through the cut point; adhesive accumulates on the knife; and adhesive transfers through the cut in the web backing and is deposited on the rotating drum.
Applying oils, greases, waxes, and lubricants to the knife, as commonly performed with lathe slitters, do not prevent these adhesive accumulation and transfer problems. Varying the knife grinding angles, dimensions, tooth type, and tooth size also did not noticeably prevent these problems. Varying the angle between the knife axis and the drum surface also did not eliminate the adhesion problems. Chilling the knife to temperatures from -50.degree. F. to 40.degree. F. eliminated adhesion problems. However, the adhesive and web contacting the knife became stiff during the contact time and could not be easily cut.
Heated knives are described in a Dienes Corporation catalog. The catalog describes two basic applications, one at a temperature suitable for simple adhesive softening, and one at very high (carbonization) temperatures. For simple adhesive softening, the Dienes score and shear knives are heated to about 158.degree. .F (70.degree. C.). Heating prevents the adhesive from sticking to the knife. The Dienes catalog deals with longitudinal slitting, rather than transverse cutting of the web. In slitting applications, knives are continuously rolled or dragged against the adhesive coated web. By softening the adhesive next to the knife with heat, the adhesive shear strength adjacent the knife is less than in the adjacent adhesive. This low strength boundary layer of adhesive next to the knife allows the material being slit to continuously wipe most of the adhesive from the knife. However, a thin layer of adhesive still remains on the knife.
Dienes also discloses a very high temperature knife called an "element" which can be heated up to 1382.degree. F. (750.degree. C.). This high temperature carbonizes the coating and backing. Dienes discusses using this element to "separate" the material and "fuse" or seal the strands on the edge of woven material to prevent unraveling, as with typewriter ribbon.
Heated knives also are used to cut plastic against a paper substrate by softening and penetrating the plastic which leaves the paper substrate uncut. Heated razor blades are used to slit PSA film tapes by softening the adhesive during cutting. However, knife temperatures approximate 200.degree. F. High temperature heated wires have been used to cut a web without an adhesive coating by melting or burning through the web. However, rather than cutting these heated wires melt through the web and do not address preventing adhesive related problems.
With adhesive coated webs, there is no known system which heats a cutting knife to temperatures between the softening and carbonization temperatures of the adhesive. Nor is there any disclosure of heating a knife which transversely cuts an adhesive web to prevent the adhesive on the web from sticking to the knife, to prevent the adhesive from accumulating on the knife, and to prevent the adhesive from transferring through the cut in the backing and depositing on the rotating drum when the web is cut against a drum.